The proposed research encompasses two areas of investigation, both of which utilize the goldfish Mauthner cell (M-cell) as a model. In the first aspect, properties of the following identified synapses are being studied with simultaneous intracellular recordings from pre- and postsynaptic elements: 1) inhibitory synapses between identified interneurons and the M-cell, and 2) mixed electrotonic and chemical excitatory synapses between vestibular afferents and the M-cell. Emphasis has been placed upon correlations of presynaptic branching patterns with transmitter release, and the mode(s) of operation of mixed synapses. Also, the convergence of M-cell collaterals and sensory inputs to the inhibitory interneurons are being determined in order to develop a model of interneuron function. The second aspect deals with axon reactions of central neurons to axotomy and deafferentation. Work to date has indicated that the M-axon does not regenerate following transection and that there is a limited axon reaction of the cell. Physiological and morphological techniques will now be utilized to analyze the effects of eighth nerve transection on M-cell properties and on the organization of its synaptic inputs. One focus of this research will be the factors contributing to a behaviorally observed M-cell hyperexcitability which appears 10-70 days postoperatively. Also, the role of inter- and intraneuronal trophic interactions in the maintenance and regulation of M-cell membrane properties and synaptic transmission will be studied. These investigations will involve the intracellular injection of labeled amino acids, nucleoside derivatives, glycoproteins and mucopolysaccharides, with emphasis placed on an analysis of dendritic transport functions. These systems will also be investigated after axotomy and deafferentation in order to correlate the trophic functions with neuronal and transneuronal responses to injury.